LIFE102, Weeks 8 & 10
LIFE102, Weeks 8 & 10 Life 102
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This 5 page Class Notes was uploaded by Sydney Dingman on Sunday March 27, 2016. The Class Notes belongs to Life 102 at Colorado State University taught by Erik N Arthun in Winter 2016. Since its upload, it has received 30 views. For similar materials see Attributes of Living Systems in Biology at Colorado State University.
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Date Created: 03/27/16
LIFE102 Week 8/10 Notes Finishing up Chapter 13: o Mitosis and Meiosis Three events are unique to meiosis, and all three occur in meiosis 1 Synapsis and crossing over in prophase 1: homologous chromosomes physically connect and exchange genetic information At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes At anaphase 1, is homologous chromosomes, instead of sister chromatids, that separate o Why does sexual reproduction exist? More genetic variation The genetic variation produced in sexual life cycles contributes to evolution Asexual reproduction: Offspring are all genetically-identical (no variation) Sexual reproduction: Offspring are genetically-different (genetic variation) Advantage of sexual reproduction: Offspring are less vulnerable to environmental change o Origins of Genetic Variation Among Offspring The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation 3 mechanisms contribute to genetic variation: o Independent assortment of chromosomes: homologous chromosomes orient randomly and are divided randomly into daughter cells in meiosis 1 o Crossing over: homologous portions of 2 non- sister chromatids trade places; homologous chromosomes pair: synapsis; pieces of DNA are exchanged, producing recombinant chromosomes o Random fertilization: adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) 3/21/16, Chapter 14: Mendelian Genetics Encoding Genetic Information o Gene: Stretch of DNA (part of a chromosome) that contains the information to make 1 protein o Genetic Traits: the physical properties of an organism determined by 1 or more genes Homologous chromosomes o 2 copies of each chromosome 2 copies of each gene one copy from each parent o Each copy encodes for the same protein, but with possible variations in sequence o Locus: the location of a gene on a chromosome o Alleles: different versions of a gene o Homologous genes: same locus on different chromosomes Different Alleles for Flower Color in Garden Peas o Gregor Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments o Genes are given to offspring via gametes o Typical experiment: Mendel mated 2 contrasting, true-breeding varieties, a process called hybridization Hybridization: forcing to make babies o The true-breeding parents are the P generation Original generation (first combined plants) o The hybrid offspring of the P generation are called the F1 generation Second generation (first babies) o When F1 individuals self-pollinate or cross-pollinate with other F1 hybrids, the F2 generation is produced 2 Third generation (second babies of first babies) Mendel’s Experiments o Crossed white- and purple-flowered pea plants: al F1 hybrids were purple o Crossed the F1 hybrids many of the F2 plants had purple flowers but some had white o Mendel discovered a ratio of about 3:1, purple to white flowers, in the F2 generation Mendel’s Models o Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids o Mendel called the purple flower color a dominant trait and the white flower color a recessive trait o The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation o Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring o Alternative versions of genes account for variates in inherited characters o The gene for flower color in pea plants exist in two versions; now called alleles o If the two alleles at a locus differ, then the dominant one determines the organism’s appearance, and the recessive allele has no noticeable effect on appearance 3/25/16, Chapter 14 cont. Law of segregation: the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes o An egg or a sperm gets only one of the two alleles that are present in the organism The possible combinations of sperm and egg can be shown using a Punnett square o A diagram for predicting the results of a genetic cross between individuals of known genetic makeup Useful Genetic Vocabulary 3 o An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character o An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character o Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition o Therefore, we distinguish an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup Mendel’s Laws of Inheritance o Law of segregation: alleles for a specific trait will separate from each other during gamete formation o Law of Independent Assortment: alleles for one trait will segregate independently of alleles for a different trait Inheritance of 2 genes o On different chromosomes: independent assortment o Far apart on the same chromosome: can be independent o Close together on the same chromosomes: linked Extending Mendelian Genetics for a Single Gene o Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations When alleles are not completely dominant or recessive (incomplete dominance) When a gene has more than 2 alleles (codominance) Degrees of Dominance o Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical Ex. The color of the purple and white Mendel’s pea flowers o Incomplete dominance occurs when the phenotype of F1 hybrids is somewhere in between the phenotypes of the two parental varieties Ex. The color of the red and white of Mendel’s pea flowers o Codominance: most genes exist in populations in more than two allelic forms and would receive a combination of the two Ex. Blood types, type AB blood 4 Polygenic Inheritance o Quantitative characters are those that vary in the population along a continuum o Quantitative variation usually indicates polygenic inheritance an additive effect of two or more genes on a single phenotype o Skin color in humans is an example of polygenic inheritance Epistasis o In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus Ex. In Labrador retrievers and many other mammals, coat color depends on two genes o One gene determines the pigment color (Black/Brown) o The other gene (with alleles E for color and e for no color) determines whether the pigment will be deposited in the hair or not 5